PM-
The rifle weighs ten pounds, the barrels (30") weigh six pounds.
It was built in 1864 by request of The Bishop for the New Bond Street shop (thanks WR for the ink-signed documents on the rifle).
At some time after the rifle was built, it was 'Converted by Fletcher of Gloucester' - likely at that time the 3-bite action and rib extension were added, also likely at that time the chambers were brought open to 3" and the barrels re-regulated and sighted for the new load.
Fletcher of Gloucester was seen by many as a 'Builder to the Gunbuilders', he had a few patents, and some very well recognized admirers and patrons - including Samuel Baker who spoke very highly of him in a few places over the years, noting that his favorite rifle of them all (that which went to bed with him every night) was built by Fletcher of Gloucester.
Neat stuff there.
I do not have shop-notes signifying the charge prescription.
I have found by way of measurement and exercise that the rifle likes groove-diameter ball (.685" or better) and the thin brass case with a charge in the neighborhood of 140+ Grains of Goex FFg or Swiss 2f or 1-1/2F
My last time out at the ranch with it I was buring Swiss 2F and 500gr ~.693 ball (~14 bore ball from Track)
The powders all tend to print to just about the same point of impact, the Goex powder leaves a much nicer *moist and manageable* fouling where the Swiss fouling is flaky and hard.
I'll focus on getting the final-final with Goex powder. There's enough room for ample greased felt wadding and Goex powder to make it run right.
Still, I'm looking to somehow (again just for the sake of a curious streak) work the load back to a period charge-prescription.
I hope that clarifies my interest in the exercise here.
This is a photo of the rifle in it's current condition.
From your conversion formula, and my bench/target experience with this rifle and Swiss 2F powder it would appear to be a "Five Dram" rifle.
Cheers
Tinker